1. Field of the Invention
The present invention relates to a light emitting transistor, more specifically, a static induction light emitting transistor comprising an organic EL element structure and a vertical FET structure.
2. Description of the Related Art
Organic electroluminescence (EL) elements have a simple element structure, and are expected as light emitting elements for next-generation displays, which are thin, light, large-sized and inexpensive. In recent years, research thereon has been actively conducted.
For driving systems for organic EL elements, it is considered that field effect transistors (FETs) of an active matrix driving system which uses thin film transistors (TFTs) are effective in working speed and power consumption. On the other hand, concerning semiconductor materials which constitute thin film transistors, in addition to researches on inorganic semiconductor materials such as silicon semiconductors and compound semiconductors, researches on organic thin film transistors (organic TFTs) using organic semiconductor materials has been actively carried out. Such organic semiconductor materials are expected as next generation semiconductor materials. However, there is a problem that they have a lower charge mobility and a higher resistance than inorganic semiconductor materials.
On the other hand, concerning static induction transistors (SITs) having a vertical FET structure, where the constructions of field effect transistors are made vertical, have advantages as follows: the channel length of the transistors can be made shorter; high-speed response and large electric power are possible since the entire electrodes on the surface can be effectively used; and they are not easily affected by interfaces.
In recent years, making use of the above-described characteristics of the static induction transistor (SIT), investigation has been made on development of a light emitting transistor, wherein this SIT structure and an organic EL element structure are combined (for example, see Japanese Patent Application Laid-Open No. 2003-324203 (claim 1), and “Present Situation and Prospect of Organic Transistors” by Kazuhiro KUDOH, Applied Physics, vol. 72, No. 9, pp 1151-1156 (2003)). FIG. 6 is a sectional structural view showing an example of a light emitting transistor wherein an SIT structure and an organic EL element structure are combined. As shown in FIG. 6, this light emitting transistor 101 has a vertical FET structure comprising: on a glass substrate 102; a source electrode 103 made of a transparent conductive film; a hole transporting layer 104 in which a slit-shaped gate electrode 105 is embedded; a light emitting layer 106; and a drain electrode 107 provided in this order. This composite type light emitting transistor 101 has a structure where the slit-shaped Schottky gate electrode 105 is embedded in the hole transporting layer 104. The hole transporting layer 104 and the gate electrode 105 form Schottky conjugation, thereby forming a depletion layer in the hole transporting layer 104. The expanse of this depletion layer varies due to gate voltage. Thus, by changing the gate voltage, the channel length is controlled. And by changing the current value between the source electrode 103 and the drain electrode 107, switching is attained.
However, the light emitting transistors, wherein an SIT structure and an organic EL element structure are combined, described in the above mentioned JP-A No. 2003-324203 and in “Present Situation and Prospect of Organic Transistors” by Kazuhiro KUDOH, Applied Physics, vol. 72, No. 9, pp 1151-1156 (2003)), have a problem that light emitted from their light emitting layer is shielded by their opaque gate electrode so that the efficiency for taking out the light is deteriorated.
Concerning such a problem, investigation that the emitted light from the light emitting layer 106 is prevented from being shielded by the opaque gate electrode 105 has been made: by making the gate electrode 105 transparent or semitransparent in the light emitting transistor 101 shown in FIG. 6, so as to prevent the shielding of the light; or by providing the slit-shaped gate electrode 105 in an electron transporting layer 108, as shown in a light emitting transistor 111 in FIG. 7.
Although these methods do not cause the problem that light is shielded by the gate electrode 105, the methods have a problem that an electric field is shielded by the gate electrode 105 placed in between the drain electrode 103 and the source electrode 107, so as to generate a shade of the electric field on the light emitting layer 106. This shade of the electric field causes a local deterioration of the light emission of the light emitting layer, so as to deteriorate the light emission efficiency thereof. As a result, these methods have a problem that the efficiency for taking out light is deteriorated.